Power source selection

ABSTRACT

A circuit for selecting between a primary power source and a back-up power source is provided in one embodiment. The circuit includes a first port configured to be coupled to a primary power source, a second port configured to be coupled to a back-up power source, a third port configured to be coupled to provide power to a load. The circuit also includes first and second power field effect transistors (FET) coupled between the second port and the third port, a third power FET coupled between the first port and the third port, and a dual ideal diode-OR controller coupled between the second and third power FETs to selectively turn on and off the second and third power FETs. The circuit further includes an opto-isolator coupled to a control input of the first power FET and a controller, coupled to the opto-isolator, that selectively turns on and off the opto-isolator.

This application is a 371 National Stage application of InternationalPatent Application No. PCT/US2017/063873 filed on Nov. 30, 2017, whichclaims the benefit of U.S. Provisional Patent Application No. 62/438,365filed on Dec. 22, 2016, the contents of both of which are herebyincorporated in their entirety.

BACKGROUND

Electronic circuits require power for proper operation. This power maybe provided by any appropriate source, such as a rectifier, a battery, asolar cell, a fuel-cell, and the like. If the power source isinterrupted or lost, the electronic circuit will cease to function. Inmany systems, users of these electronic circuits expect that the systemwill continue to function at all times. Therefore, many electronicsystems also provide a backup power source that is connected to theelectronic circuit in the event of a failure in the primary powersource.

There are many conventional approaches for switching between the primarypower source and the backup power source for an electronic circuit. Forexample, some systems use mechanical relays to switch from the primarypower source to backup power when the primary power source becomesunavailable. Other systems use field effect transistors (FET) only basedsolid-state switches, or wired or ideal diode-OR components. Many ofthese systems use a break-then-make switching technology whichinterrupts the power to load for the switching interval, and/orexperience voltage level limitations, slow response times and hightransients.

Therefore, there is a need in the art for an improved circuit forswitching between a primary power source and a backup power source.

SUMMARY

A circuit for selecting between a primary power source and a back-uppower source is provided in one embodiment. The circuit includes a firstport configured to be coupled to a primary power source, a second portconfigured to be coupled to a back-up power source, a third portconfigured to be coupled to provide power to a load. The circuit alsoincludes first and second power field effect transistors (FET) coupledbetween the second port and the third port, a third power FET coupledbetween the first port and the third port, and a dual ideal diode-ORcontroller coupled between the second and third power FETs toselectively turn on and off the second and third power FETs. The circuitfurther includes an opto-isolator coupled to a control input of thefirst power FET and a controller coupled to the opto-isolator thatselectively turns on and off the opto-isolator. The controller monitorsthe power received at the first port and, when the power at the firstport crosses a first threshold level, turns on the opto-isolator so thatpower is transmitted by the first and second power transistors betweenthe second port and the third port and when the power at the first portcrosses a second threshold level, turns off the opto-isolator so thatpower is transmitted by the third power transistor between the firstport and the third port.

DRAWINGS

Understanding that the drawings depict only exemplary embodiments andare not therefore to be considered limiting in scope, the exemplaryembodiments will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is a block diagram of one embodiment of a circuit for switchingbetween a primary power source and a backup power source according tothe teachings of the present invention.

FIG. 2 is a flowchart of one embodiment of a method for switchingbetween a primary power source and a backup power source according tothe teachings of the present invention.

FIG. 3 is a block diagram of an embodiment of an electronic systemincluding N circuits for switching between N primary power sources, eachassociated with a respective one of the N circuits, and a backup powersource that is shared by the N circuits.

FIG. 4 is a block diagram of an embodiment of an electronic system thatincludes a circuit for selecting between a primary power source and abackup power source according to the teachings of the present invention.

FIG. 5 is a block diagram of another embodiment of an electronic systemthat includes a circuit for selectively switching to one backup powersource in place of one of N primary power sources to provide power toone of N respective loads.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments. However, it is tobe understood that other embodiments may be utilized and that logical,mechanical, and electrical changes may be made. Furthermore, the methodpresented in the drawing figures and the specification is not to beconstrued as limiting the order in which the individual steps may beperformed. The following detailed description is, therefore, not to betaken in a limiting sense.

Embodiments of the present invention provide a capability to externallyswitch load power from a “primary” power source to a “backup” powersource and from a “backup” power source to a “primary” power sourcewithout interruption to the operation of the load. In one embodiment, acircuit automatically detects a drop in primary power voltage andswitches the load power input to a backup power source when the primarypower source falls below a configurable threshold level. In anotherembodiment, the circuit automatically detects a rise in primary powervoltage and switches load power from the backup power source back to theprimary power source once the primary power source rises above adifferent, configurable threshold level. The power switching iscompleted in a smooth and fast manner such that the load does notexperience sufficient voltage drop or current transients that wouldcause it to cease operating. This switching is accomplished independentof the relative voltage levels of the two power sources provided thebackup voltage level is greater than the primary's falling threshold.

For pedagogical purposes, this specification generally describes theembodiments being connected to positive input and output voltage levels.It is understood, however, that other embodiments of this inventionfunction in typical telecommunication applications that use negativevoltages (e.g., −48V). For embodiments using negative input and loadvoltages, the terms “fall” or “drop” associated with an input voltagewould indicate the voltage is going less negative; the term “rise”associated with a negative input voltage would indicate the voltage isgoing more negative. In light of this dual embodiment, the voltageslevels as discussed in this specification are best viewed as absolute(rather than positive or negative) values.

FIG. 1 is a block diagram of one embodiment of a circuit, indicatedgenerally at 100, for switching between a primary power source and abackup power source according to the teachings of the present invention.The circuit 100 includes two ports for receiving power. Primary powerport 116 is configured to be coupled to a primary power source. Primarypower port 116 has two nodes labelled Primary and Primary return (RTN),respectively. Backup power port 120 is adapted to be coupled to a backuppower source and includes two nodes labelled Backup and Backup RTN,respectively. The circuit 100 also includes a port that is configured toprovide power to a load. Load power port 124 includes two nodes labelledLoad Power and Load RTN.

The circuit 100 includes two paths for providing power to the load. Thefirst (primary) path includes power field effect transistor (FET) 105coupled between primary power port 116 and load power port 124. Thesecond path (back-up) includes power field effect transistors (FETs) 101and 102 that are coupled in series between backup power port 120 andload power port 124. Advantageously, use of power transistors provideslower heat dissipation than diodes or mechanical relays used inconventional approaches due to the low inline “on” resistance of thepower MOSFETS. Power MOSFETs are also smaller in size than mechanicalcomponents that carry equivalent current. Further, embodiments of thepresent invention provide enhanced reliability and faster response timesresulting from using solid state technology rather than mechanicalcomponents.

The circuit 100 also includes a control circuit for switching betweenthe primary power source and the backup power source. This controlcircuit includes microcontroller 110, opto-isolator 103 anddual-ideal-OR controller 104. Opto-isolator 103 is coupled betweenmicrocontroller 110 and a control input of FET 101. Microcontroller 110provides control signals to turn on and off opto-isolator 103 asdescribed in more detail below. Opto-isolator 103 bridges the gapbetween low voltage electronics in the microcontroller 110 and thehigher voltage regime of the power FETs, e.g., FETs 101 and 102. Dualideal diode-OR controller 104 is coupled to FET 102 and FET 105.Controller 104 alternatively turns on and off the power FETs 102 and 105as described in more detail below.

Microcontroller 110 determines when to switch between the primary powersource at primary power port 116 and the backup power source at backuppower port 120. The microcontroller 110 accomplishes this by comparingthe voltage of the primary power source at primary power port 116against two thresholds, high threshold 107 and low threshold 109, asdescribed in more detail below. In another embodiment, themicrocontroller 110 could be replaced by discrete analog and/or digitallogic performing the same functionality.

Circuit 100 also includes voltage and current sensing circuit 114.Voltage and current sensing circuit 114 gathers data on voltage andcurrent in circuit 100 through current sense elements 150, 152, and 154.Current sense element 150 measures current from back-up power port 120.Current sense element 152 measures current from primary power port 116.Finally, current sense element 154 measure current at load power port124. Voltage and current sensing can also be used to monitor power andexpended energy. Circuit 114 allows the voltage and current levels ofthe backup and primary power sources to be monitored by microcontroller110. Communication between voltage and current sensing circuit 114 andmicrocontroller 110 is accomplished by way of a two-wire interface.Communication schemes in other embodiments include Serial PeripheralInterface (SPI), Universal Serial Bus (USB) or circuit 114 can haveanalog outputs that connect to an analog to digital converter (ADC)internal to the microcontroller 110. In one embodiment, the voltage andcurrent sensing circuit 114 can be used to detect faults, such as,overvoltage, under voltage, over current, low current, or the like.

Low threshold 109 and high threshold 107 used by microcontroller 110 maybe adjusted through microcontroller 110 for a specific implementation.In one embodiment, the high and low voltage thresholds 107 and 109 areset by way of a two-wire interface connecting a variable voltage dividerto microcontroller 110. In another embodiment, the high and lowthresholds 107 and 109 are set by references 106 and 108 respectively.These embodiments for thresholds 107 and 109 could be implemented via adiscrete reference voltage, a digital potentiometer, stored memory or,for fixed thresholds, a highly precise resistor array. This pathprovides a mechanism for setting the window to implement hysteresis asdiscussed in more detail below that prevents switching oscillation andinstability.

Microcontroller 110 includes a communication (Comm) port 112. Comm Port112 provides an interface to an external host that allows for thecommunication, monitoring and control of the voltages, currents andswitching thresholds in circuit 100. Comm port 112 could be used tochange switching thresholds 107 and 109, create an alarm when aswitching event occurs, or provide feedback about the primary sourcevoltage and current levels. This could be implemented as serial data(e.g., RS-232, RS-485), Ethernet, or discrete digital input/outputlines. Advantageously, comm port 112 enables field site adjustments andreal time monitoring of voltage and current levels to an external hostnot provided by current art. This comm port 112 allows for remotemonitoring of voltage and current levels, e.g. at the base when circuit100 is installed at the tower top.

Circuit 100 also includes power converter 113. Power converter 113converts high voltage levels at, for example, load power port 124 to oneor more lower level voltages needed by microcontroller 110 and othercontrol functions in circuit 100. In other embodiments, power converter113 may receive high voltage level input from primary power port 116 orback-up power port 120. In one embodiment, power converter 113 alsoincludes a battery 128 that provides power to circuit 100 when no poweris output at load power port 124. In this way, power converter 113 canprovide power to microcontroller 110 and other low voltage circuitry(for example, voltage and current sensing circuit 114, high threshold107, low threshold 109, reference 106, and reference 108) from battery128 to configure and control circuit 100 in the absence of inputs atboth primary power port 116 and backup power port 120 or failure of loadport 124. In another embodiment, power converter 113 can also selectbetween power inputs 120 and 116.

FIG. 2 is a flowchart on one embodiment of a method for switchingbetween a primary power source and a backup power source according tothe teachings of the present invention. The operation of circuit 100will be described in conjunction with the process of FIG. 2. The processof FIG. 2 is divided into two paths at block 202. At block 202, it isdetermined whether the primary or back up power source is currentlyproviding power for the load. If the primary power source is providingpower, the process proceeds to block 204 to determine if the primarypower source needs to be replaced by the backup power source. Otherwise,if the back-up power source is providing power to the load, the processproceeds to block 210 to determine if the primary power source is backon-line.

According to this process, circuit 100 normally supplies power to theload from the primary source. To accomplish this, microcontroller 110turns opto-isolator 103 off which in turn keeps FET 101 off, physicallydisconnecting the backup power source from the load. When circuit 100 isin this condition as determined at block 202, the microcontroller 110compares the primary source voltage to a first threshold, e.g., the lowthreshold 109 voltage value at block 204. This comparison determineswhether the primary voltage is present and sufficient to power the load.If the primary power source has not crossed the first threshold, forexample, has not dropped below the low threshold, then the processreturns to block 200 and, with the backup source physicallydisconnected, the dual ideal diode-OR controller 104 enables FET 105 todrive power to the load from the primary power source. The processcontinues to monitor the primary power source at block 200.

A short, brown-out, other fault condition or deactivation may occur tothe primary power source which causes its voltage level to cross thefirst threshold, e.g., fall below low threshold 109. This fallingvoltage level is detected by microcontroller 110 at block 204 and theprocess proceeds to optional block 206.

In an alternate embodiment discussed below, a backup power source isshared between N primary power sources. In such an embodiment, thecircuit 100 does not switch to the backup power source if another of theN primary power sources has already been replaced by the backup powersource. Thus, the process returns to block 200 and monitors the primarypower source.

If however, the backup power source is not in use or the backup is notshared, the process proceeds to block 208. The Microcontroller 110reacts by turning on opto-isolator 103 to enable power FET 101 once thefirst threshold is crossed, e.g., the voltage drops below the lowthreshold. Once enabled, power FET 101 allows voltage to pass to powerFET 102. Since the backup voltage level will be higher than that of theprimary voltage at its low threshold, the dual ideal diode-OR controller104 will detect current flowing through the body diode of power FET 102and switch load power to the backup by turning off power FET 105. It isnoted that the design of circuit 100 has the benefit that the backupvoltage can be higher or lower than the primary voltage without beingswitched to the load when not needed. This is not the case with an idealdiode-OR only solution that simply switches whichever voltage is thehighest to the load.

This control mechanism provides a number of additional benefits overconventional circuits used to switch between primary and backup powersources. For example, circuit 100 provides smooth switching betweenpower sources while reducing voltage and current transients. Largeenergy spikes associated with normal power switching are eliminated.Further, fast switching response time enabled by controller 110,opto-isolator 103 and FET 101 allows load power to be switched beforethe primary voltage level drops below the minimum required load voltage.This avoids a power loss to the load that would interrupt loadoperation. Additionally, embodiments of the invention act as a voltageprioritizer for voltages higher (up to 100V) than current prioritizers(up to 36V).

The process returns to block 200 and monitors the primary power source.Circuit 100 continues to supply load power from the backup once theprimary source crosses the first threshold, e.g., falls below lowthreshold 109 until the primary source voltage level crosses a secondthreshold, e.g., rises above a high threshold 107. This techniqueimplements hysteresis such that switching oscillation does not occurfrom a slow slew rate, noise or transients on the primary voltage as itis falling below low threshold 109. The use of programmable switchingthresholds with inherent hysteresis facilitates easy and relatively wideadjustable thresholds and hysteresis to accommodate system noise andswitching transients. Averaging and digital filtering could also beemployed for a more robust hysteresis algorithm. Current art depends onresistor dividers or fixed values for threshold and hysteresis.

If the fault condition is cleared from the primary power source or theprimary power source is reactivated causing its output voltage to rise,microcontroller 110 will detect this rising voltage as it crosses highthreshold 107 at block 210. Once microcontroller 110 detects the primaryvoltage rising above high threshold 107, it proceeds to block 212 andshuts off opto-isolator 103 which in turn disables power FET 101. Thiseffectively disconnects the backup power source to the load once again.Dual diode-OR controller 104, sensing current beginning to flow throughthe body diode of power FET 105 and current ebbing with voltage droppingand finally reversing in power FET 102, will enable power FET 105 anddisable power FET 102. Load power has now been shifted back to theprimary power source from the backup power source. The process returnsto block 200 and monitors the primary power source.

Hysteresis protection against switching oscillation is also implementedonce circuit 100 switches load power back to the primary by preventinganother switch to the backup unless the primary voltage level once againfalls below low threshold 109 using the process described above withrespect to blocks 202, 204, 206, and 208.

FIG. 3 is a block diagram of an embodiment of an electronic system,indicated generally at 300, including N circuits (302-1 to 302-N) thatshare one backup power source between N loads. Circuits 302-1 to 302-Neach switch between their respective primary power source at primarypower ports 306-1 to 306-N and a shared backup power source at backuppower port 304. Advantageously, this embodiment of the invention enablesbackup switching for multiple, N, loads from a single backup powersource or 1:N redundancy. This feature allows the backup power source atbackup power port 304 to be switched to any one of “N” output loads atload power ports 308-1 to 308-N. Since the backup power source typicallyis insufficient to power multiple loads, the backup power sourceprovides power to only one of the “N” loads at a time. In thisembodiment, the switching of a backup source to one and only one of Nmultiple loads concurrently is accomplished through an “open drain”(drive low only) signaling technique as described below.

For 1:N redundancy configurations such as shown in FIG. 3, one powerselection circuit 302 is needed per load. In one embodiment, the powerselection circuit is implemented using the circuit 100 of FIG. 1. In atypical embodiment, individual, primary power sources are connected tothe primary power ports 306-1 to 306-N of system 300. For backup power,the single, backup power port 304 couples the backup power source to thebackup power source inputs of the power selection circuits 302.

Each power selection circuit 302 has a 1:N open-drain (OD) control I/Osignal that is connected to all the power selection circuits 302. The ODcontrol I/O signal is both an input and output of microcontroller 110and serves to communicate the state of the backup control logic of theentire system 300. The driver for this signal in microcontroller 110(FIG. 1) can either be disabled (“tri-stated”) or driven low dependingupon the state of the backup switch logic for all the individual powerselection circuits. This OD control I/O signal is also wired back tomicrocontroller 110 as an input for monitoring the state of the ODcontrol I/O signal. Monitoring can be accomplished with control logicimplemented internal or external to the microcontroller 110.

During normal operation, the individual power selection circuits 302-1to 302-N provide power to their loads from the individual primary powersources coupled to primary power ports 306-1 to 306-N, respectively. Thebackup source is physically disconnected in each of the power selectioncircuits 302-1 to 302-N and does not provide power to the power loadnode 308-1 to 308-N coupled to the respective power selection circuits302-1 to 302-N as described above. Under these conditions, each powerselection circuit 302-1 to 302-N will disable its OD control I/O driverand resistor 111 will pull this signal to a logic “high” indicating toall power selection circuits 302-1 to 302-N that no individual powerselection circuit 302-1 to 302-N has switched to the backup powersource.

In this redundancy embodiment, power selection circuits 302-1 to 302-Ninclude the optional block 206 of FIG. 2. The function of block 206 isgoverned by the current state of the OD control I/O signal. At block204, if microcontroller 110 detects the voltage level of the primarysource falls below low threshold 109, then microcontroller 110 checksthe state of the OD control I/O signal at block 206. If it is “high” asdescribed above, microcontroller 110 not only activates opto-isolator103 as described above with respect to block 208 but also drives its ODcontrol I/O signal low. This communicates to all of the other powerselection circuits 302 that one of the power selection circuits 302-1 to302-N has switched to use the backup power source. If the state of theOD control I/O signal is low at block 206, then microcontroller 110 isprevented from activating its opto-isolator 103 at block 208 that wouldcause the output load to switch to the backup power source due to thefact that one of the other individual power selection circuits 302 hasalready switched its load to the backup power source.

Should the primary source voltage level of the redundant system that isswitched to backup power rise back above high threshold 107 (block 210),then microcontroller 110 again “tri-states” the OD control I/O signal(at block 212). Resistor 111 will pull the OD control I/O signal backhigh allowing any of the power selection circuits 302 to switch tobackup if their primary power source falls below low threshold 109.

FIG. 4 is a block diagram of an electronic system, indicated generallyat 400, that includes a circuit 406 for selecting between a primarypower source 402 and a backup power source 404 according to theteachings of the present invention. Circuit 406 is coupled to load 408,for example, telecommunications circuitry such as a remote radio head,remote unit of a distributed antenna system, or other appropriateelectronic circuit. Circuit 406 also includes battery 412 to providepower to the circuit 406 to enable configuration and control of circuit406 when primary power source 402 and backup power source 404 are notavailable or the output power port to load 408 has failed. Circuit 406also includes comm port 410 to provide an interface for configuringcircuit 406 in a similar manner to embodiments discussed above. Circuit406 is configured to include in-line power switches controlled bylow-voltage circuitry and an ideal-OR controller to enable fast, smoothswitching to backup power source 404 when primary power source dropsbelow a configurable threshold. Circuit 406 implements hysteresis toprevent switching oscillation from a slow slew rate, noise or transientson the primary voltage as it is falling below the threshold. In oneembodiment, circuit 406 is configured as shown and described above withrespect to FIG. 1.

In operation, circuit 406 selects between primary power source 402 andbackup power source 404. When primary power source crosses a firstthreshold, e.g., drops below a selected voltage, circuit 406 selectsbackup power source 404 and passes this power to load 408. When thebackup power source 404 is providing power to load 408, circuit 406monitors primary power source 402 to determine when the primary powersource 402 is back on-line. Circuit 406 determines when the primarypower source 402 crosses a second threshold, e.g., rises above a secondselected voltage. When this occurs, circuit 406 switches back to usingprimary power source 402 as the power source for the load 408.

FIG. 5 is a block diagram of another embodiment of an electronic system,indicated generally at 500, that includes a circuit 506 for selectivelyapplying one backup power source 504 in place of one of N primary powersources 502-1 to 502-N to one of N respective to loads 508-1 to 508-N,for example, telecommunications circuitry such as a remote radio head,remote unit in a distributed antenna system or other appropriateelectronic circuits. Circuit 506 also includes battery 512 to providepower to the circuit 506 to be used to provide power to circuit 506 toenable configuration and control of circuit 506 if load power isunavailable due to power output failure or if primary and backup powersources are not available. Circuit 506 also includes comm port 510 toprovide an interface for configuring circuit 506. In one embodiment,power source selection circuit 506 is constructed as shown and describedabove with respect to FIGS. 1 and 3.

In operation, circuit 506 selects between primary power sources 502-1 to502-N and backup power source 504. When one of the primary power sources502-1 to 502-N crosses a first threshold, e.g., drops below a selectedvoltage level, circuit 506 selects backup power source 504 and passesthis power to the corresponding load 508. Circuit 506 also sets a signalthat indicates that the backup power source 504 is currently in use.This signal is used to prevent the backup power source 504 from beingswitched to any of the other N loads. When the backup power source 504is providing power to one of N loads 508-1 to 508-N, circuit 506monitors the primary power source 502 that was switched out to determinewhen the primary power source 502 is back on-line. Circuit 506determines when the primary power source 502 is back online when thepower supplied by the primary power source 502 crosses a secondthreshold, e.g., rises above a second selected voltage. When thisoccurs, circuit 506 switches back to using primary power source 502 asthe power source for the corresponding one of loads 508-1 to 508-N andclears the signal indicating backup power is available.

EXAMPLE EMBODIMENTS

Example 1 includes a circuit for selecting between a primary powersource and a back-up power source. The circuit includes a first portconfigured to be coupled to a primary power source, a second portconfigured to be coupled to a back-up power source, a third portconfigured to be coupled to provide power to a load, first and secondpower field effect transistors (FET) coupled between the second port andthe third port, a third power FET coupled between the first port and thethird port, a dual ideal diode-OR controller coupled between the secondand third power FETs to selectively turn on and off the second and thirdpower FETs, an opto-isolator coupled to a control input of the firstpower FET, a controller, coupled to the opto-isolator, that selectivelyturns on and off the opto-isolator, wherein the controller monitors thepower received at the first port and, when the power at the first portcrosses a first threshold level, turns on the opto-isolator so thatpower is transmitted by the first and second power transistors betweenthe second port and the third port and when the power at the first portcrosses a second threshold level, turns off the opto-isolator so thatpower is transmitted by the third power transistor between the firstport and the third port.

Example 2 includes the circuit of example 1, wherein the controllerincludes a port that produces a signal that enables the circuit to sharethe backup power source in a 1:N redundancy arrangement.

Example 3 includes the circuit of any of examples 1 and 2, wherein thefirst and second thresholds have different, configurable values.

Example 4 includes the circuit of example 3, wherein the controllerturns on the opto-isolator when the power at the first port, as measureby a voltage level at the first port, drops below a low voltagethreshold.

Example 5 includes the circuit of example 4, wherein the controllerturns off the opto-isolator when the power at the first port, asmeasured by a voltage level at the first port, crosses above a highvoltage threshold that is above the low voltage threshold.

Example 6 includes the circuit of any of examples 1-5, and furthercomprising a voltage and current sensing circuit configured to sense atleast the voltage or current at at least one of the first, second andthird ports.

Example 7 includes the circuit of any of examples 1-6, and furthercomprising a communications port coupled to the controller that isconfigured to establish the first and second thresholds.

Example 8 includes the circuit of any of examples 1-7, and furtherincluding at least one of a discrete reference voltage, a digitalpotentiometer, stored memory or a highly precise resistor array that areconfigured to establish the first and second thresholds.

Example 9 includes the circuit of any of examples 8, and furthercomprising a power converter that is coupled to receive a high inputvoltage from at least one of the first, second and third ports andconvert the voltage to one or more lower level voltages for use by atleast the controller.

Example 10 includes the circuit of example 9, wherein the powerconverter further includes a battery port that is configured to becoupled to a battery to provide power to the controller and other lowvoltage devices in the absence of a voltage at the first, second andthird ports.

Example 11 includes a system that includes a load, a primary power portconfigured to be coupled to a primary power source, a back-up power portconfigured to be coupled to a back-up power source, a power sourceselection circuit, coupled to the load and the primary and back-up powerports. The power source selection circuit includes at least one powerfield effect transistor in a first path between the primary power portand the load, at least two power field effect transistors in a secondpath between the back-up power port and the load, a voltage and currentsensing circuit, and a controller, coupled to one of the power fieldeffect transistors in the first path and the voltage and current sensingcircuit, wherein the controller is configured to selectively connect theback-up power source to the load by turning on and off the one of thepower field effect transistors in the first path in response to theoutput of the voltage and current sensing circuit.

Example 12 includes the system of example 11, wherein the load comprisesone of telecommunications circuitry, a remote radio head, remote unit orother circuitry in a distributed antenna system.

Example 13 includes the system of any of examples 11 and 12, and furthercomprising a dual diode-OR controller configured to selectively turn onand off the at least one power field effect transistor in the first pathand the other of the at least two power field effect transistors in thesecond path.

Example 14 includes the system of any of examples 11-13, and furtherincluding a communications port coupled to the controller, thecommunications port configured to receive inputs that establishthresholds used by the controller to determine when to turn on and offthe one of the power field effect transistors in the first path.

Example 15 includes the system of any of examples 11-14, wherein theload comprises a plurality of loads, the primary power port comprises aplurality of primary power ports, each of the primary power portsassociated with a corresponding one of the plurality of loads, and thepower source selection circuit selectively connects the back-up powersource to one of the plurality of loads in response to a sensedcondition of the corresponding primary power source.

Example 16 includes a method for selecting a power source for a load.The method includes monitoring the primary power source, when theprimary power source is providing power to the load, determining if acondition of the primary power source crosses a first threshold, whenthe condition crosses the first threshold, turning on a first powerfield effect transistor to couple a back-up power source to the loadthrough a second power field effect transistor, when the primary powersource is not providing power to the load, determining if a condition ofthe primary power source crosses a second threshold, when the conditioncrosses the second threshold, switching off the first power field effecttransistor to couple the primary power source to the load through athird power field effect transistor.

Example 17 includes the method of example 16, wherein monitoring theprimary power source comprises monitoring a voltage level of the primarypower source.

Example 18 includes the method of example 17, wherein determining if acondition of the primary power source crosses a first thresholdcomprises determining when a voltage of the primary power source dropsbelow a low voltage threshold.

Example 19 includes the method of example 18, wherein determining if acondition of the primary power source crosses a second thresholdcomprises determining when a voltage of the primary power source risesabove a high voltage threshold that is higher than the low voltagethreshold.

Example 20 includes the method of any of examples 16-19, wherein turningon the first power field effect transistor comprises turning on thefirst power field effect transistor with an opto-isolator.

Example 21 includes the method of any of examples 16-20, and furthercomprising determining if the back-up power source is providing power toanother load prior to turning on the first power field effecttransistor.

Example 22 includes a system for providing sharing a common back-uppower source for N loads. The system includes a back-up power portconfigured to be coupled to the common back-up power source, a pluralityof primary power ports configured to be coupled to N primary powersources, a plurality of power selection circuits, each coupled to thecommon back-up power source and at least one of the plurality of primarypower ports, a plurality of load ports, each coupled to one of theplurality of power selection circuits and configured to be coupled toprovide power to one of the N loads, a control line, coupled to each ofthe plurality of power selection circuits, to communicate when one ofthe N power selection circuits is coupling the back-up power source toits load. Each of the power selections circuits includes a first portconfigured to be coupled to one of the N primary power sources, a secondport configured to be coupled to the common back-up power source, athird port configured to be coupled to provide power to one of the Nloads, first and second power field effect transistors (FET) coupledbetween the second port and the third port, a third power FET coupledbetween the first port and the third port, a dual ideal diode-ORcontroller coupled between the second and third power FETs toselectively turn on and off the second and third power FETs, anopto-isolator coupled to a control input of the first power FET, and acontroller, coupled to the opto-isolator, that selectively turns on andoff the opto-isolator. The controller monitors the power received at thefirst port and, when the power at the first port crosses a firstthreshold level, turns on the opto-isolator so that power is transmittedby the first and second power transistors between the second port andthe third port and when the power at the first port crosses a secondthreshold level, turns off the opto-isolator so that power istransmitted by the third power transistor between the first port and thethird port.

A number of embodiments of the invention defined by the following claimshave been described. Nevertheless, it will be understood that variousmodifications to the described embodiments may be made without departingfrom the spirit and scope of the claimed invention. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A circuit for selecting between a primary powersource and a back-up power source, the circuit comprising: a first portconfigured to be coupled to a primary power source; a second portconfigured to be coupled to a back-up power source; a third portconfigured to be coupled to provide power to a load; first and secondpower field effect transistors (FET) coupled between the second port andthe third port; a third power FET coupled between the first port and thethird port; a dual ideal diode-OR controller coupled between the secondand third power FETs to selectively turn on and off the second and thirdpower FETs; an opto-isolator coupled to a control input of the firstpower FET; a controller, coupled to the opto-isolator, that selectivelyturns on and off the opto-isolator; wherein the controller monitors thepower received at the first port and, when the power at the first portcrosses a first threshold level, turns on the opto-isolator so thatpower is transmitted by the first and second power transistors betweenthe second port and the third port and when the power at the first portcrosses a second threshold level, turns off the opto-isolator so thatpower is transmitted by the third power transistor between the firstport and the third port.
 2. The circuit of claim 1, wherein thecontroller includes a port that produces a signal that enables thecircuit to share the backup power source in a 1:N redundancyarrangement.
 3. The circuit of claim 1, wherein the first and secondthresholds have different, configurable values.
 4. The circuit of claim3, wherein the controller turns on the opto-isolator when the power atthe first port, as measure by a voltage level at the first port, dropsbelow a low voltage threshold.
 5. The circuit of claim 4, wherein thecontroller turns off the opto-isolator when the power at the first port,as measured by a voltage level at the first port, crosses above a highvoltage threshold that is above the low voltage threshold.
 6. Thecircuit of claim 1, and further comprising a voltage and current sensingcircuit configured to sense at least the voltage or current at at leastone of the first, second and third ports.
 7. The circuit of claim 1, andfurther comprising a communications port coupled to the controller thatis configured to establish the first and second thresholds.
 8. Thecircuit of claim 1, and further including at least one of a discretereference voltage, a digital potentiometer, stored memory or a highlyprecise resistor array that are configured to establish the first andsecond thresholds.
 9. The circuit of claim 1, and further comprising apower converter that is coupled to receive a high input voltage from atleast one of the first, second and third ports and convert the voltageto one or more lower level voltages for use by at least the controller.10. The circuit of claim 9, wherein the power converter further includesa battery port that is configured to be coupled to a battery to providepower to the controller and other low voltage devices in the absence ofa voltage at the first, second and third ports.
 11. A system,comprising: a load; a primary power port configured to be coupled to aprimary power source; a back-up power port configured to be coupled to aback-up power source; a power source selection circuit, coupled to theload and the primary and back-up power ports, wherein the power sourceselection circuit includes: at least one power field effect transistorin a first path between the primary power port and the load; at leasttwo power field effect transistors in a second path between the back-uppower port and the load; a dual ideal diode-OR controller, coupled tothe at least one power field effect transistor in the first path and afirst of the at least two power field effect transistors of the secondpath; a voltage and current sensing circuit; and a controller, coupledto a second one of the power field effect transistors in the second pathand the voltage and current sensing circuit, wherein the controller isconfigured to selectively connect and disconnect the back-up powersource to and from the load by turning on and off, respectively, thesecond one of the power field effect transistors in the second paththrough an opto-isolator in response to the output of the voltage andcurrent sensing circuit.
 12. The system of claim 11, wherein the loadcomprises one of telecommunications circuitry, a remote radio head,remote unit or other circuitry in a distributed antenna system.
 13. Thesystem of claim 11, and further comprising a dual diode-OR controllerconfigured to selectively turn on and off the at least one power fieldeffect transistor in the first path and the other of the at least twopower field effect transistors in the second path.
 14. The system ofclaim 11, and further including a communications port coupled to thecontroller, the communications port configured to receive inputs thatestablish thresholds used by the controller to determine when to turn onand off the one of the power field effect transistors in the first path.15. The system of claim 11, wherein: the power source selection circuitcomprises a plurality of power source selection circuits; the loadcomprises a plurality of loads; the primary power port comprises aplurality of primary power ports each coupled to a corresponding one ofa plurality of primary power sources, each of the primary power portsand each of the plurality of loads associated with a corresponding oneof the plurality of power source selection circuits; and each of theplurality of power source selection circuits is configured toselectively connect the back-up power source to its associated one ofthe plurality of loads in response to a sensed condition of thecorresponding one of the plurality of primary power sources.
 16. Amethod for selecting a power source for a load, the method comprising:monitoring a primary power source; when the primary power source isproviding power to the load, determining if a condition of the primarypower source crosses a first threshold; when the condition crosses thefirst threshold, turning on a first power field effect transistorthrough an opto-isolator, under control of a dual ideal diode-ORcontroller, coupling a back-up power source to the load through a secondpower field effect transistor and decoupling the primary power sourcethrough a third power field effect transistor; when the primary powersource is not providing power to the load, determining if a condition ofthe primary power source crosses a second threshold; when the conditioncrosses the second threshold, switching off the first power field effecttransistor through the opto-isolator such that the dual ideal diode-ORcontroller couples the primary power source to the load through thethird power field effect transistor.
 17. The method of claim 16, whereinmonitoring the primary power source comprises monitoring a voltage levelof the primary power source.
 18. The method of claim 17, whereindetermining if a condition of the primary power source crosses a firstthreshold comprises determining when a voltage of the primary powersource drops below a low voltage threshold.
 19. The method of claim 18,wherein determining if a condition of the primary power source crosses asecond threshold comprises determining when a voltage of the primarypower source rises above a high voltage threshold that is higher thanthe low voltage threshold.
 20. The method of claim 16, and furthercomprising determining if the back-up power source is providing power toanother load prior to turning on the first power field effecttransistor.
 21. A system for providing sharing a common back-up powersource for N loads, the system comprising: a back-up power portconfigured to be coupled to the common back-up power source; a pluralityof primary power ports configured to be coupled to N primary powersources; a plurality of power selection circuits, each coupled to thecommon back-up power source and at least one of the plurality of primarypower ports; a plurality of load ports, each coupled to one of theplurality of power selection circuits and configured to be coupled toprovide power to one of the N loads; a control line, coupled to each ofthe plurality of power selection circuits, to communicate when one ofthe N power selection circuits is coupling the back-up power source toits load; wherein each of the power selections circuits comprises: afirst port configured to be coupled to one of the N primary powersources; a second port configured to be coupled to the common back-uppower source; a third port configured to be coupled to provide power toone of the N loads; first and second power field effect transistors(FET) coupled between the second port and the third port; a third powerFET coupled between the first port and the third port; a dual idealdiode-OR controller coupled between the second and third power FETs toselectively turn on and off the second and third power FETs; anopto-isolator coupled to a control input of the first power FET; acontroller, coupled to the opto-isolator, that selectively turns on andoff the opto-isolator; wherein the controller monitors the powerreceived at the first port and, when the power at the first port crossesa first threshold level, turns on the opto-isolator so that power istransmitted by the first and second power transistors between the secondport and the third port and when the power at the first port crosses asecond threshold level, turns off the opto-isolator so that power istransmitted by the third power transistor between the first port and thethird port.